fix lock inversion between evalLock and dispatch_once on globalStreams

[davidkoski]

Proposed changes

• globalStreams could take the evalLock
• evalLock could take the globalStreams lock (dispatch_once)
• break the cycle by making the no-args init lock free

Note: this is potentially a problem on main / latest tag. The triggering condition is narrow, so not likely, but still reachable.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[davidkoski]

The problem occurred because RealizedStreamPromise would synchronously obtain a stream, which could require the evalLock to create a new stream. No problem normally, but this is the initializer that is used for:

let globalStreams = StreamPairPromise()

This in turn takes a dispatch_once lock.

Lock inversion: EvalLock -> dispatch_once and dispatch_once -> EvalLock.

Switching to the LazyStreamPromise does not acquire any resources at creation time so this does not use EvalLock under the dispatch_once (at the cost of a lock every time it is accessed ).

[nwh]

👍